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SCIENCE OF SYNTHETIC TURF SURFACES:A COMPARISON OF TRACTION TEST METHODS

STARSS CONFERENCE 201022/04/10

KATHRYN SEVERN

SCIENCE OF SYNTHETIC TURF SURFACES: PRESENTATION OVERVIEW

INTRODUCTION

MATERIALS

METHODS

•FIFA Rotational Traction Test

•Instrumented Test Device 1

•Instrumented Test Device 2

RESULTS

•Rotational Traction

•Translational Traction

•Stud Penetration

•Vertical Load

SUMMARY

ACKNOWLEDGEMENTS

SCIENCE OF SYNTHETIC TURF SURFACES: INTRODUCTION

TRACTION

Many sports utilise cleats or studs. During sports movements these cleats orstuds penetrate and interlock with the playing surface generating traction forces.

Dependent on:

• The sports specific movement

• Sports specific footwear

• The playing surface

• The environment

Since the introduction of synthetic turf surfaces, many have speculated that thesesurfaces have increased the rate of injury in players due to high levels of traction.

Limited number of studies that have attempted to quantify specific injuriesdirectly related to traction behaviour of synthetic turf surfaces

• Large and complex number of variables involved

• Resources required for comprehensive testing and injury surveillance study


THE PLAYING SURFACE

Synthetic turf surfaces have seen many developments and improvements in qualitysince their introduction into the market.

3rd GENERATION/3G SYNTHETIC TURF SURFACES•Long pile carpet, with less denselypacked fibres, 35 mm - 65 mmin length

•Inclusion of an elastomeric(rubber crumb in most cases)infill material

•Large variety of surface compositions exist

Recognised by both the international governing bodies of football and rugby,Fédération Internationale de Football Association (FIFA) and the InternationalRugby Board (IRB), for which these surfaces were predominantly design for.

Fibre

Rubber CrumbSand

Carpet Backing

Shockpad

SCIENCE OF SYNTHETIC TURF SURFACES: INTRODUCTIONTESTING

Many sports governing bodies have developed test procedures and performancerequirements to that a surface must achieve before that sport can be played onthe surface to:

• maximise performance• minimise risk of injury• Reduce differences between pitch systems

Traction is no exception:– For example FIFA state that rotational traction must fall between 25 – 50

Nm for a 1 Star pitch.

Test procedures have been designed to be:– Portable for field testing– Simple to use– Highly repeatable

This does however, lead to questions over their biomechanical representationand sensitivity, including the current standard traction (FIFA, 2009) test used tomeasure rotational traction.


TESTING

Many research institutes and sports footwear companies have developed their

own bespoke versions of mechanical tests to measure either rotational or

translational traction or in some cases both in order to better replicate player

movements:

• Sports specific footwear

• Varying the vertical (static) load

• Hydraulically driven apparatus

• Imitations of the lowers limbs

Such variations in the methodologies used have made it difficult to compare

historical data.

This presentation reports on a data set comparing traction behaviour of carpetand rubber infill systems using three different test procedures.

20 kg/m212 kg/m27 kg/m24 kg/m2SBR Rubber Crumb0.5 – 1.5 mm

10 kg/m210 kg/m210 kg/m210 kg/m2Silica Sand0.2 – 0.7 mm

67200268800453600127200Total Number of Fibres

per sqm

0.0260.0260.0250.025Fibre Thickness

(mm)

1 – 31.31.51.5Fibre Width (mm)

2 for each fibre (whichthen fibrillate)

81812No. of Fibres per Tuft

33600336002520010600No. of Tufts per sqm

65 (Main fibrillated fibre)504035Pile Height (mm)

2 types of fibrillated fibre,one curly (to help hold

infill)MonofilamentMonotapeMonofilamentFibre Type

PolyethylenePolyethylenePolyethylenePolypropyleneFibre Material

DCBASurface

SCIENCE OF SYNTHETIC TURF SURFACES: MATERIALS

SCIENCE OF SYNTHETIC TURF SURFACES: METHODOLOGY

LOOSECOMPACTED

FIFA Standard Test Instrumented Test Device 1 Instrumented Test Device 2

Test Apparatus

Traction Measurement Rotational Translational Both

Operator Manual Automated Automated

Displacement Minimum of 90 degrees ofrotation

200 mm 60 degrees of rotation

50 mm

Speed 12 revolutions per minute N/A 12 degrees per second

10 mm per second

Vertical load Free weights Hydraulic ram Free weights

Footwear/Base Round metal disc withstandard FIFA studs attached

Plastic sole shaped disc withstandard FIFA studs attached

Actual football boot withstuds similar in dimension to

FIFA standard

46 mm

46 mm

29 mm

37 mm

11 mm

60 mm

30 mm

40 mm30 mm

55 mmStud Configuration

SCIENCE OF SYNTHETIC TURF SURFACES: METHODOLOGY

COMPACTED

LOOSE

TEST PROGRAMME

•Four surface systems, with a 30 mm shockpad•Four surface conditions were created for each surface system using a standard 30kg studded roller to change the state of the rubber infill layer; raked (i.e. loose), 50rolls, 100 rolls and 200 rolls.

Infill height was measured using a standardpenetration gauge, and infill density was calculatedfor each condition.

Rotational and/or translational traction was measuredfor each surface system with the different devices ata vertical load of 450 N (or a close as possible).

Effect of varying the vertical load:•FIFA (160 N, 260 N, 360 N and 460 N)•Test Device 1 (250 N, 350 N and 450 N)•Test Device 2 (450 N, 550 N and 650 N)

The values chosen were dependent on the mechanics and capabilities of thedevices.

SCIENCE OF SYNTHETIC TURF SURFACES: RESULTS

0.670.600.540.573020137200 Rolls

0.650.550.470.503122158100 Rolls

0.590.500.430.47342416.58.550 Rolls

0.540.430.370.4037282010Raked

Carpet DCarpet CCarpet BCarpet ACarpet DCarpet CCarpet BCarpet ASurfaceCondition

Net Bulk Density of Rubber Crumb (g/cm3)Rubber Crumb Infill Height (mm)

RUBBER INFILL DENSITY

•Four surface conditions were created using a weighted studded roller to compactthe infill material.

•The level of compaction and resulting bulk density of the infill material isdependent to an extent on the quantity of infill used and the carpet fibre in eachsurface system.

Infill height and bulk density of the rubber crumb from compaction with a studded roller

Number of Fibres per sqm

Carpet A Carpet B Carpet C Carpet D

127200 453600 268800 67200


FORCE/DISPLACEMENT HISTORY

Test Device 2: Both rotational and translational traction history traces showed threekey regions.

1 – Sharp increase in resistance

2 – Graduated increase until a peak is reached

3 – Decrease in resistance following the peak

1 2 3

0

100

200

300

400

500

600

700

0 10 20 30 40 50 60

Horizontal Displacement (mm)

Ho

rizo

nta

lF

orc

e(N

)

Peaktranslationaltraction

1 2 3

Peakrotationaltraction

0

5

10

15

20

25

30

0 10 20 30 40 50 60

Angular Displacement (Degrees of Rotation)

Ro

tati

on

alT

racti

on

(Nm

)

1 2 3

FORCE/DISPLACEMENT HISTORY

Test Device 1: Demonstrates the effect of vertical load on peak translationaltraction

-100

0

100

200

300

400

500

600

700

0 50 100 150 200 250

Displacement (mm)

Fo

rce

(N)

Horizontal Force_1 Vertical Force_1

-100

0

100

200

300

400

500

600

700

800

900

1000

0 50 100 150 200 250

Displacement (mm)

Fo

rce

(N)

Horizontal Force_1 Vertical Force_1


ROTATIONAL TRACTION

FIFA: Good repeatability but no trend with increasing the initial bulk density of the

rubber infill layer or carpet systems.

Device 2: Increase in rotational traction with initial bulk density of the rubber infill layer.

The effect of initial rubber infill bulk density on the peak rotational traction

18

23

28

33

38

0.3 0.4 0.5 0.6 0.7Initial Rubber Infill Bulk Density (g/cm3)

Peak

Ro

tati

on

alT

racti

on

(Nm

)

Carpet A FIFA Carpet B FIFA Carpet C FIFA Carpet D FIFACarpet A Boot A Carpet B Boot A Carpet C Boot A Carpet D Boot A

FIFA

Test Device 2

Large difference in peak rotational traction values between the two devices.


ROTATIONAL TRACTION

Large difference in peak rotational traction values between the two devices.

Variations between the two devices:

•Number of studs

•Stud spacing

•Outsole material

•Speed and distance of rotation

FIFA StandardTest

InstrumentedTest Device 2

TractionMeasurement

Rotational Both

Operator Manual Automated

Displacement Minimum of 90degrees of

rotation

60 degrees ofrotation

50 mm

Speed 12 revolutionsper minute

12 degrees persecond

10 mm persecond

Vertical load Free weights Free weights

Footwear/Base Round metal discwith standard

FIFA studsattached

Actual footballboot with studs

similar indimension toFIFA standard

0

5

10

15

20

25

30

35

40

A B C DCarpet

Peak

Ro

tati

on

al

Tra

cti

on

(Nm

)

100

105

110

115

120

125

130

135

140

Fo

rce

per

Stu

d(N

)

46 mm 23 mm 46 mm 23 mm

•The effect of studspacing on the peakrotational traction usingthe FIFA test device at astud spacing of 46 mmand 23 mm from thecentral pivot point.

46 mm

46 mm

30 mm

40 mm30 mm

55 mm

FIFA Test Device 2

Pivot Point


TRANSLATIONAL TRACTION

Device 2: Increase in translational traction with an increase in the initial bulk density

of the rubber infill layer.

Device 1: Peak translational traction difficult to interpret due to poor repeatabilityand variations in vertical force. Measurements taken at 10 mm ofdisplacement found to have better repeatability, but no differences werefound between surfaces and did not correlate with Test Device 2.

1.000

1.100

1.200

1.300

1.400

1.500

0.4 0.45 0.5 0.55 0.6 0.65 0.7Initial rubber Infill Bulk Density (g/cm3)

Co

eff

icie

nt

of

Tra

ns

lati

on

al

Tra

cti

on



STUD PENETRATION

•A reduction in stud penetration was found as the thickness (mass) of the rubber infilllayer increased.

•Increasing the vertical load of the generally showed an increase in stud penetration.

•Full penetration of a FIFA standard stud (13 mm length) was found with all carpetssuggesting compression of the surface layer occurred.

•No pattern was found between stud displacement and initial bulk density of the rubberinfill layer.

LOOSECOMPACTED

0

5

10

15

20

25

30

35

40

45

50

A B C D

Carpet

Ve

rtic

alD

isp

lac

em

en

t/D

ep

th(m

m)

Rubber Sand

Vertical Load250 N350 N450 NStud Length


VERTICAL LOADING

•An increase in traction was shown with all three test devices with an increase in thevertical load applied to the surface.

•A non-linear relationship was observed as the vertical load increased.

The effect of vertical load on peak rotational traction with Test Device 2.

18.00

20.00

22.00

24.00

26.00

28.00

30.00

32.00

443 543 643Vertical Load (N)

Pe

ak

Ro

tati

on

al

Tra

cti

on

(Nm

)


SCIENCE OF SYNTHETIC TURF SURFACES: SUMMARY

Rotational and translational traction behaviour of four carpet and rubber infill systemshas been measured using three different test devices.

The surface composition and conditioning applied had an effect on the density of therubber infill.

Peak rotational traction measured with the standard FIFA test showed little differencebetween the four surface systems or with varying net bulk density of the rubber infill.

Device 2 showed an increase in both rotational and translational traction for anincrease in the initial net bulk density of the rubber infill.

Modifying the standard test procedures demonstrated that traction at the shoe-surfaceinterface is greatly influenced by changes in the normal force applied and spacing ofstuds from the central pivot point for peak rotational traction.

Stud penetration (Device 1) was measured and was found to be influenced by verticalload, though no real conclusions were reached on these effects on the translationaltraction measured.

SCIENCE OF SYNTHETIC TURF SURFACES: RECOMMENDATIONS

These comparisons between test devices and surface systems suggest that moreresearch into traction behaviour of systems is clearly warranted

To better understand traction mechanisms under player loading, traction relatedresearch needs to incorporate both more surface materials behaviour anddevelop test apparatus that control loads and measure penetration/compression.

Test Device Wish List:• Use of real boot, or a boot shape with the ability to adapt the studs• Control of vertical loads• Vary the loading rate• Measure the stud penetration• Measure compression of the surface under loading• Measure the whole force-displacement history• Portable for field testing• Programmable with feedback control to permit simulation of different player

manoeuvres

This is an ambitious wish list, but coupled with careful sample selection andpreparation, is considered essential to better determine the mechanisms oftraction at the shoe-surface interface.

SCIENCE OF SYNTHETIC TURF SURFACES: ACKNOWLEDGEMENTS

ACKNOWLEDGEMENTS

• The Adidas Test Centre in Germany

• The University of Sheffield (James Clarke and Matt Carré)

For their support for some of the testing work for this study

Thank You

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